1. Field of the Invention
The present invention relates to a mixer for mixing signals and, in particular, the present invention relates to a digital mixer for converting a signal from a frequency band into another frequency band, wherein the digital mixer may, for example, be used in telecommunications.
2. Description of the Related Art
In telecommunications, to shift a signal from a current frequency (current frequency) into a higher transmission frequency (target frequency), mainly mixers are used. For such a shifting, for example in the transmitter several different possibilities are possible. First, a signal having a low bandwidth Blow may be shifted to different center frequencies within a large bandwidth B. If this center frequency is constant over a longer period of time, then this means nothing but the selection of a subband within the larger frequency band. Such a proceeding is referred to as “tuning”. If the center frequency to which the signal is to be shifted varies relatively fast, such a system is referred to as a frequency-hopping system or a spread-spectrum system. As an alternative, also within a large bandwidth B several transmission signals may be emitted in parallel in the frequency multiplexer with a respectively low bandwidth Blow.
Analog to these proceedings in the transmitter, the respective receivers are to be implemented accordingly. This means on the one hand that a subband of the large bandwidth B is to be selected when the center frequency of the transmitted signal is constant over a longer period of time. The tuning is then performed to the predetermined center frequency. If the center frequency is varied relatively fast, as it is the case with a frequency-hopping system, also in the receiver a fast temporal change of the center frequency of the transmitted signal has to take place. If several transmit signals have been sent out in parallel in the frequency multiplexer, also a parallel reception of those several frequency-multiplexed signals within the larger bandwidth B has to take place.
Conventionally, for an above-indicated tuning system and a frequency-hopping system an analog or digital mixer is used, wherein the digital mixing conventionally takes place with one single mixer stage. In an analog mixer, a high expense in circuit technology is necessary, as for a precise mixing to the target frequency highly accurate mixer members are required which substantially increase the costs of the transmitter to be manufactured. It is to be noted with regard to a digital mixer that in certain respects a high expense in terms of circuit engineering (or numerics, respectively) is required when the signal is to be mixed onto a freely selectable random target frequency.
Conventional mixers may here be implemented similar to the mixer device 2400, as it is illustrated in FIG. 24 in the form of a downsampling mixer. The mixer device 2400 includes a mixer 2402, a low-pass filter 2404 and a sampling rate mixer 2406. The mixer 2402 comprises an input 2408 for receiving a signal 2410 to be mixed. Further, the mixer 2402 comprises an output 2412 for outputting the signal 2414 converted from the current frequency to an intermediate frequency which is supplied to the low-pass filter 2404 via an input 2416 of the same. Further, the low-pass filter 2404 comprises an output 2418 for outputting a frequency-converted low-pass-filtered signal 2420 which is supplied to the sampling rate mixer 2406 via an input 2422 of the same. The sampling rate mixer 2406 includes an output 2424 for outputting a sampling rate-converted signal 2426 which is simultaneously the output signal output from the mixer device 2400.
If now the start signal 2410 having the current frequency is supplied to the mixer device 2400, wherein the start signal 2410 is based on a first sampling frequency defined by an interval of two time-discrete signal values, a conversion of the current frequency to an intermediate frequency is performed by the mixer 2402, whereupon the intermediate frequency signal 2414 results. In this intermediate frequency signal 2414, however, only the frequency on which the start signal 2410 is located (i.e. the current frequency) is converted to an intermediate frequency; the sampling frequency was not changed by the mixer 2402. In a suitable selection of the current frequency and the sampling frequency, in a simple way with regard to numerics or circuit engineering, a mixing onto the intermediate frequency signal 2414 having the intermediate frequency may be realized. If the spectral interval between the current frequency and the intermediate frequency of the (complex) signal 2410 is a quarter of the sampling frequency regarding its magnitude, then a mixing may be performed by a multiplication with the values 1, i, −1 and −i, or even more simply, merely by a negation of real part or imaginary part values of the start signal 2410 as well as by an exchange of real and imaginary part values of signal values of the start signal 2410.
Subsequently, a low-pass filtering of the intermediate frequency signal 2414 with the first sampling frequency is performed by the low-pass filter 2404, whereupon a low-pass-filtered intermediate frequency signal 2420 results which is again based on the first sampling frequency. By the sampling rate mixer 2406, a downsampling of the low-pass-filtered intermediate frequency signal 2402 takes place. This leads, for example, to a reduction of the sampling frequency without further spectrally converting the signal. An above-described mixer 2402 which is simple to be implemented regarding numerics or hardware engineering is, for example, described in Marvin E. Frerking, Digital Signal Processing in Communication Systems, Kluwer Academic Publishers.
Such an approach of a mixer 2402 which may easily be realized in terms of numerics or circuit engineering has the disadvantage that by the predetermined connection between the current frequency and the sampling frequency only intermediate frequencies may be obtained which are arranged in a spectral interval of a quarter of the sampling frequency around the current frequency. This reduces the applicability of such a mixer 2402 which may efficiently be realized in terms of numerics or circuit engineering. If also intermediate frequencies are to be obtained, which comprise another interval to the current frequency than a quarter of the sampling frequency, a multiplication of the individual start signal values of the start signal 2410 with the rotating complex pointer ej2πkfc/fs is necessary, wherein k is a running index of the start signal values, fc is the desired center frequency (i.e. the intermediate frequency) and fs is the sampling frequency of the signal. It is to be considered, however, that in the multiplication of the start signal values with the rotating complex pointer not only purely real or purely imaginary multiplication factors, respectively, are to be used, but that the multiplication factors used comprise real and imaginary parts. By this, a solution efficient in terms of numerics and circuit engineering, as it was indicated above, may not be used. A mixer would be desired, however, which offers the possibility to perform a mixing of start signal values from a current frequency to any intermediate frequency in an efficient way in terms of numerics and circuit engineering.
For the parallel transmitting and receiving of several frequency subbands, frequently the OFDM (orthogonal frequency division multiplexing) and the related multi-carrier modulation or multi-tone modulation method, respectively, are used. The same require, by the use of the Fourier transformation, partially a substantial computing expenditure, in particular if only few of the frequency subbands from a large frequency band having many individual frequency subbands are required.